1. Field of the Invention
The present invention relates to high-frequency complex components and mobile communication devices including the same, and more particularly, the present invention relates to a high-frequency complex component obtained by connecting a receiving-side LC parallel resonant-type filter to a surface acoustic wave filter in a cascade arranged and to a mobile communication device including the same.
2. Description of the Related Art
In Japan, a PDC (Personal Digital Cellular) method operating in the 800 MHz band or the 1.5 GHz band is presently adopted in mobile communication devices and corresponding frequency bands are reserved for mobile transmission and station transmission. That is, the transmission band and the reception band of mobile devices are reserved, so that simultaneous transmission and reception are performed using one channel for the transmission band and one channel for the reception band. The interval between the operating channel for transmission and the operating channel for reception is controlled so as to be always constant. At this time, most duplexers, which are used for sharing one antenna for transmission and reception, often have a construction, in which a dielectric filter is used, as proposed in Japanese Unexamined Patent Application Publication No. 9-83214.
FIG. 8 shows a block diagram of a conventional duplexer disclosed in Japanese Unexamined Patent Application Publication No. 9-83214. A duplexer 50 includes a branch circuit 51, a receiving-side dielectric filter 52 having a dielectric coaxial resonator, a surface acoustic wave filter 53, and a transmitting-side dielectric filter 54 having a dielectric coaxial resonator. The branch circuit 51, the receiving-side dielectric filter 52, and the surface acoustic wave filter 53 are connected between a first terminal 501 and a second terminal 502. The branch circuit 51 and the transmitting-side dielectric filter 54 are connected between the first terminal 501 and a third terminal 503. In such a construction, an antenna ANT is connected to the first terminal 501, the second terminal 502 is connected to a reception circuit Rx, and the third terminal 503 is connected to a transmission circuit Tx.
However, according to the conventional duplexer, use of a dielectric filter causes the duplexer to become large and adjustment of the central frequency becomes difficult. As a result, an increase in the size of a mobile communication device and deterioration of the characteristics thereof become problems.
This is because the length of the dielectric filter is equal to xcex/4, where xcex is the wavelength of a reception signal or the wavelength of a transmission signal. For example, the length of the filter is 5 cm in the 1.5 GHz band and it is as long as 9.375 cm in the 800 MHz. Although the dielectric element constituting the dielectric filter must be chiseled in order to adjust the central frequency of the dielectric filter, it is very difficult to apply fine adjustment to the dielectric.
In order to overcome the problems described above, preferred embodiments of the present invention provide a duplexer which is easy to miniaturize and in which the central frequency is easily adjusted, and also provide a mobile communication device including such a novel duplexer.
According to a first preferred embodiment of the present invention, a duplexer preferably includes a branch circuit connected to an antenna and arranged to branch to a receiving side and a transmitting side, a receiving-side LC parallel resonant-type filter connected to the receiving-side of the branch circuit and arranged to cause a reception signal to be passed and a transmission signal to be attenuated, a surface acoustic wave filter connected to the receiving-side LC parallel resonant-type filter defining a subsequent stage of the receiving-side LC parallel resonant type filter and arranged to cause the reception signal to be passed and the area of the high-frequency side of the reception signal to be attenuated, and a transmitting-side LC parallel resonant-type filter connected to the transmitting-side of the branch circuit and arranged to cause the transmission signal to be passed and the reception signal to be attenuated.
Use of an LC parallel resonant-type filter enables the duplexer to be easily adapted to the frequency of the reception signal or the frequency of the transmission signal by changing the values of the inductors and the capacitors without changing the sizes or the appearances of the inductors and the capacitors that constitute the LC parallel resonant-type filter. Therefore, miniaturization of the duplexer is facilitated along with the adjustment of the central frequency.
The branch circuit may include a balun element.
Since a branch circuit preferably includes a balun element, both of a terminal connected to a transmission circuit observed from a terminal connected to a reception circuit and the terminal connected to the reception circuit observed from the terminal connected to the transmission circuit are open in terms of a high frequency. Therefore, since complete isolation is obtained between these terminals, the reception signal is prevented from interfering with the transmission signal side, and vice versa. Consequently, the reliability of the duplexer is improved.
A duplexer may further include a multi-layer substrate defined by a laminated body having a plurality of dielectric layers. In the duplexer, the branch circuit, the receiving side LC parallel resonant-type filter, and the transmitting side LC parallel resonant-type filter may be integrated in the multi-layer substrate and the surface acoustic wave filter may be mounted on the multi-layer substrate.
A multi-layer substrate is preferably formed by laminating a plurality of dielectric layers, the multi-layer substrate which the branch circuit, a receiving-side LC parallel resonant-type filter, and a transmitting-side LC parallel resonant-type filter are integrated therein and a surface acoustic wave filter is mounted thereon. Accordingly, matching adjustment can be easily performed between the branch circuit and the receiving-side LC parallel resonant-type filter, between the receiving-side LC parallel resonant-type filter and the surface acoustic wave filter, and between the branch circuit and the transmitting side LC parallel resonant-type filter. This eliminates the necessity of matching circuits that perform corresponding matching adjustment. Therefore, the duplexer can be further miniaturized. In addition, loss due to wiring between the branch circuit and the receiving-side LC parallel resonant-type filter, between the receiving-side LC parallel resonant-type filter and the surface acoustic wave filter, and between the branch circuit and the transmitting-side LC parallel resonant-type filter can be minimized. Therefore, the overall loss of the duplexer is greatly minimized.
The connection of each of the branch circuit, the receiving-side LC parallel resonant-type filer, the surface acoustic wave filter, and the transmitting-side LC parallel resonant-type filter can be provided and contained within the multi-layer substrate. Therefore, further miniaturization of the duplexer is achieved.
According to a second preferred embodiment of the present invention, a mobile communication device includes a duplexer according to the first preferred embodiment of the present invention.
Since the duplexer which can be easily miniaturized and which can have easily the central frequency adjusted is included, a miniaturized and high-performance mobile communication device can be obtained.
Other features, elements, characteristics and advantages of the present invention will become apparent from the following detailed description of preferred embodiments with reference to the attached drawings.